Remove Shopify purchase options when a preorder offer is disabled

Currently, when a STOQ preorder offer is disabled, the Shopify Purchase Option / Selling Plan created for that offer can remain attached to the products that were previously included in it.

Because the product is still associated with a Shopify purchase option, the storefront/theme can continue treating it differently even though the preorder itself is no longer active. This affected the accelerated checkout behavior, and the normal Buy with Shop experience only returned after the product was manually removed from the disabled preorder offer.

We need this feature so that disabling a preorder offer fully returns the affected products to their normal Shopify purchase state without requiring merchants to manually remove products from old offers.

When an offer is disabled, STOQ should automatically remove its Shopify Purchase Option / Selling Plan association from products or variants that are no longer part of any active preorder offer. Products still covered by another active preorder offer should remain unchanged.